Means for measuring or detecting the position of a control rod within a nuclear reactor are limited by the fact that the measurement needs to be made within the primary water for the nuclear reactor.
A conventional method for determining the relative location of a control in a nuclear reactor is to use a metallic probe tube which extends into the primary water region, and which houses a coil of wire forming an inductive element that forms part of an electrical circuit.
The probe tube is positioned such that a metallic leadscrew attached to the control rod moves telescopically over the probe tube as the control rod is moved in and out of the nuclear reactor to regulate the fission reaction therein.
As the leadscrew moves over the probe tube the voltage across the inductor changes because of magnetic coupling effects. This change in voltage is directly proportional to the position of the leadscrew and thus the control rod.
A problem with using this method is that it is typically not very accurate. In particular, it has a low span to offset ratio and a low signal span. This is problematic because the measurement instrumentation is typically limited to relatively low signal voltages, and it is thus desirable to maximise the signal span to offset ratio so that the relative position of the leadscrew (and therefore the control rod) can be known with high accuracy.
A further problem with the prior art techniques is that the flux density of the field that is generated around the inductive element is difficult to predict before manufacture. It is common practice, therefore, to manufacture a multitude of inductive elements, the one with the best magnetic field in terms of the spread of the flux ultimately being selected for use.
Indeed, each element may need to be calibrated in situ, so that variations in the local operating environment can be accounted for in the calibration. This is undesirable.
Some prior art methods of measurement use the transformer principle rather than the simple inductor principle. The transformer principle also involves a metallic probe tube and a metallic leadscrew, but the probe tube houses a series of transformer windings alternating between electromagnetically coupled primary and secondary windings along a core. When in operation, a magnetic field is generated between the primary and secondary windings. As the leadscrew moves over the probe tube the magnetic field between the windings is affected such that the voltage generated across the secondary windings changes proportionately to the position of the leadscrew over the probe tube.
An example of a transformer effect sensor is U.S. Pat. No. 5,563,922, which shows the use of a transformer effect to sense the moving metallic item through a metallic enclosure. However, in the arrangement shown in U.S. Pat. No. 5,563,922, the output signal typically suffers from a low span to offset ratio. As mentioned above is undesirable because it reduces the sensitivity of the sensor and therefore the accuracy to which the relative position of the leadscrew (and therefore the control rod) can be known.
In particular, in arrangements similar to that of U.S. Pat. No. 5,563,922, the signal span is relatively small. And, typically, a large residual magnetic field exists between the primary and secondary windings when the leadscrew is “covered” (i.e. the leadscrew is arranged to cover the probe tube). This typically results in a large voltage offset on the output signal of the sensor, which is undesirable.
In particular, when an output signal is amplified the voltage offset of the signal is also amplified, which causes difficulty for subsequent signalling processing of the output signal; indeed, it can make it difficult to detect the relevant part of the signal, because it is swamped by the amplified offset level (and any associated noise on the offset level).